In a Code-Division Multiple Access (CDMA) cellular communication system, a key component in determining power consumption, performance and cost in the User Equipment (UE) is the Power Amplifier (PA). Non linearity of the PA can cause significant impairment to the quality of the transmitted signal. This effect increases when the PA is run close to its maximum power rating. With current technology, the approach taken to mitigate this problem is to back off the PA (i.e. lower its power) from the region where its input-output characteristic is markedly non linear. At lower powers, the PA input-output characteristic is closer to linear, and hence the backoff causes the PA to operate in a region in which less distortion due to non linearity is introduced.
In modern communications systems, the trend is towards a system designed to handle many different communication types related to different types of applications—for example SMS, voice, video, email, web browsing. These different types of communications are usually based on specific and different transmission formats. The traffic mix to be transmitted by a given user can be changing with time—for example a speech call may be established, and part way through the call, a packet video upload may take place. In certain communication systems, the characteristics of the transmitted signal may vary with time also because of the automatic adaptation of the transmission format based on the propagation channel and traffic conditions. These time varying signal characteristics determine, in turn, the degradation caused by the non linearity in the PA.
A parameter often used to quantify the effect of the signal characteristics on the required power backoff is the Peak to Average Ratio (PAR), defined as the ratio between the peak signal power and average signal power. A signal with higher PAR is likely to suffer more degradation from a given PA operating at a given (average) power than one with lower PAR.
A particular problem faced in the standardisation of 3GPP Wideband CDMA (WCDMA) systems was how to design the uplink in such a way that Adjacent Channel Leakage Ratio (ACLR) is kept under control. ACLR is defined in 3GPP TS 25.101 as a measure of the power leakage into an adjacent carrier, taking into account both transmit and receive Root-Raised Cosine (RRC) filtering. ACLR is one of the important impairments in WCDMA. A primary cause of ACLR is the third order nonlinearity of the PA's transfer characteristic. In 3GPP TSG RAN WG1 #37 Tdoc R1-040642 (“Comparison of PAR and Cubic Metric for Power De-Rating”, May 2004) a signal characteristic called the Cubic Metric (CM) is defined, which is strongly related to the PA backoff required to pass a WCDMA signal without degradation. The WCDMA standard has been designed to allow a backoff in the uplink PA maximum power, which can be a function of the cubic metric.
One approach for choosing the power backoff of a PA is given in US patent application no. US2007/0155335 (Robert T. Love, Richard C. Burbidge, Edgar P. Fernandes and Vijay Nangia, “Method and Apparatus for Power Reduction for E-TFC Selection”). This discloses a technique for determining the required back-off from a look-up table of different possible combinations of weighting factors for the different WCDMA channels.